Does anyone have bottom and top entrance in their hives ?
I do. I use a medium entrance reducer on the bottom and a 3/4" hole on the top.
I been told the bees will only guard one entrance. Having two entrances,will that be an invite to robbers and all other pests ?
Bees will guard as many entrances as you have. I have one hive that has an entrance reducer that I made with two 3/4"x 3/8" openings (1 on each side), a crack in one of the boxes the bees come and go from, and a 3/4" hole on top. They guard them all. I will say that robbing gets them pretty crazy though.
Would it be better to just have one top entrance ?
Some say it is. In my opinion it's really up to you though.
With top entrances,do you put landing boards on ?
No. Bees don't really need a landing board. I've never seen a feral hive with a landing board.